Brightest of them all

Astronomers have recently glimpsed the most powerful supernova ever seen. A star in a galaxy billions of light years away exploded with such force, that it briefly shone nearly 600 billion times brighter than our sun. It challenges all known theories of supernovae.

Supernova is a phenomenon where a very bright star like object appears suddenly in the sky, fades away in a few months and eventually becomes invisible to the naked eye. Some of the ancient civilisations, especially the Chinese, kept a record of these events. Four supernovae visible to the naked eye have been well recorded in the last millennium. The most well known of them is the Crab supernova, which appeared in the sky on July 4, 1054.

It was learnt in the last century that supernovae represent the end products of the death of a star. Fusion reaction where four hydrogen atoms come together to create a helium nucleus and lot of energy makes a star start shining. The pressure due to this reaction opposes the gravitational force, which tries to contract the star and keeps the star intact. In big stars, this fusion process goes on till the element iron is produced. At this stage, the balance between pressure and gravity is upset, and the star begins to contract into a very small volume.

The net result is the formation of a small but dense star (neutron star) in the stellar core and the total disruption of the remainder of the star with the liberation of enormous energy (equivalent to what the sun would release in its lifetime) in neutrinos and photons. These myriad photons are responsible for the brightness of a supernova. However, the fact that it continues to be bright for several months is due to the decay of radioactive nucleii produced in the star. This is called a Type II supernova. When a white dwarf star in a binary star system accretes material from its companion star and its mass exceeds the Chandrasekhar Mass Limit (1.44 solar mass), it will collapse and produce a Type Ia supernova. Type I and II supernovae can be distinguished by the way they dim with time and their spectra.

One of the most famous supernovae of recent times seen by the naked eye appeared on February 23, 1987. This supernova, termed SN1987A, was seen in the Large Magellanic Cloud, a nearby galaxy. While the discovery of pulsars in 1967 had already proved the existence of neutron stars, this was the first time neutrinos were detected from a supernova. Thus, observation of this supernova at various steps confirmed several of the earlier theories on supernovae. On an average, only one supernova per galaxy per century is expected to take place. Till recently, the supernova of 2006 was considered to be the brightest ever observed.

In recent times

An innocuous dot of light in an image of the southern skies was spotted on June 15, 2015. The spectrum of this object taken later showed a very high red shift indicating that it a had taken place very far away, some 3.8 billion light years away. This showed that this was a very, very bright supernova and was termed ASASSN-15lh, named after the All Sky Automated Survey for Supernovae. It was found to be 20 times brighter than the combined light of Milky Way, making it the brightest supernova ever observed.

After several further observations, this was reconfirmed in January 2016. If the supernova took place in our own galaxy, it would be easily seen by the naked eye even during the day; If it was at a distance of about nine light years, it would be as bright as the sun. If it were as close as Pluto, it would vapourise the whole solar system. This supernova is two to three times brighter than the biggest one which appeared in 2006. According to some theorists, this supernova could really be the brightest that ever can be.

The electromagnetic spectrum of a star tells us not only about its distance, but also about the elements that are present. The progenitor of this supernova is thought to be a massive, blue, hot star, rotating rapidly. It must have shed its outer layers of hydrogen and helium shortly before it died, because those elements are absent from the spectrum.

There are two most likely explanations for such a huge energy output. The supernova could be due to the death of a very big star. An example is Eta Carinae which is a hyper giant star (about 130 times more massive than sun) located approximately 7,500 light years from Earth. If Eta Carinae exploded in a similar fashion, it would be bright enough that one could read by its light here during night, and would even be visible during the daytime.

Another explanation is that it is due to a magnetar, which is the strongest magnet known in the universe. At ~ 1015 Gauss, the magnetic field is a thousand trillion times stronger than that of Earth. They would probably have to spin at a rate of 1,000 times a second to provide this huge amount of energy via a magnetised wind, which could produce enough shocks to emit this enormous burst of light. Magnetars are formed in the same way as other neutron stars, but with the core collapse of much more massive stars. We will get more information about this supernova when the Hubble telescope will be used for its study in the next few months.